Method for recovering a frame that failed to be transmitted in a mu-mimo based wireless communication system

ABSTRACT

A method and apparatus of transmitting in a wireless communication system is provided. A transmitter generates a group management frame including a group information and position information and a data frame including a group control information having a group identifier for each of a plurality of groups. The group information indicates whether the receiver is a member of each of the plurality of groups. The position information indicates a position of at least one stream corresponding to at least one group of which the receiver is a member.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. patent applicationSer. No. 13/479,165 filed on May 23, 2012, which is a continuation ofPCT/KR2010/008355, filed on Nov. 24, 2010, which claims the benefit ofKorean Patent Application No. 10-2010-0046119, filed on May 17, 2010,Korean Patent Application No. 10-2010-0023423, filed on Mar. 16, 2010,and Korean Patent Application No. 10-2009-0113869, filed on Nov. 24,2009, which are hereby incorporated by reference in their entirety intothis application.

TECHNICAL FIELD

Exemplary embodiments of the present invention relate to a method forrecovering a transmission-failed frame in wireless communication system;and, more particularly, to a method for recovering a frame, which hasfailed to be transmitted, among frames transmitted to a plurality ofwireless stations using a group number in a wireless communicationsystem based on MU-MIMO (Multi-User Multiple Input Multiple Output).

BACKGROUND ART

A WLAN basically supports a BSS (Basic Service Set) consisting of an AP(Access Point), which serves as the point of access of a DS (DistributedSystem), and a plurality of wireless STAs (STAtions) other than the AP.

The MAC (Medium Access Control) protocol of the WLAN operates based onCSMA/CA (Carrier Sense Multiple Access/Collision Avoidance). Therefore,the WLAN involves resource wasting in the course of channel contention.In order to alleviate this problem, IEEE 802.11 Workgroup ‘e’ hasdefined an enhanced MAC protocol, which proposes that, when a radioresource transmission right is acquired, multiple MPDUs are transmittedusing SIFS (Short Inter-Frame Space) during a TXOP (TransmissionOpportunity), and block ACKs are received in response thereto (i.e.burst transmission).

Furthermore, IEEE 802.11 Workgroup ‘n’ has defined an A-MSDU (AggregatedMSDU) and an A-MPDU (Aggregated MDPU). At least one MSDU (which is atransmission unit) or at least one MPDU is aggregated even without IFS,and is transmitted in the process of one-time radio resource contention.

Recent surge in the number of WLAN users is followed by standardizationregarding VHT (Very High Throughput) WLAN systems by IEEE 802.11Workgroup ‘ac’ as an attempt to increase data throughput provided byeach BSS.

A VHT WLAN system supports, in a multiple (three)-STA environmentconsisting of one AP and two STAs, maximum throughput of 1 Gbps at theMAC SAP of the AP, as well as maximum throughput of 500 Mbps at the MACSAP of a wireless STA for point-to-point environments. There is alsoconsideration made so that each AP and STA of the VHT WLANsimultaneously supports compatibility with existing WLAN (IEEE 802.11a/nsystem).

Meanwhile, when a wireless STA acquires a TXOP in a wirelesscommunication system (e.g. WLAN), the STA needs to receive a responsethrough a response frame regarding a request frame in order to improvereliability of wireless communication. For example, frames requiringresponses include a RTS (Request-To-Send) frame, a BAR (Block ACKRequest) frame, a data frame, and frames for various requests (e.g.probe request, authentication request, association request). Responseframes responding to the frames requiring responses are as follows: aresponse frame responding to a RTS frame is a CTS (Clear-To-Send) frame;a response frame responding to a BAR frame is a BA (Block ACK) frame; aresponse frame responding to a data frame is an ACK or BA frame; andresponse frames responding to various request frames are frames ofvarious responses (e.g. probe response, authentication response,association response).

The responses are classified into immediate responses and delayedresponses. Responses (ACK frames) to a single piece of data correspondto the immediate responses; and block responses responding to acontinuous transmission or aggregated MPDU correspond to both immediateand delayed responses.

An immediate response is used in the following manner: whenPHY-RXEND.primitive of a received request frame is generated, a responseframe is transmitted after SIFS so that other wireless STAs do nottransmit. In this case, the frame exchange sequence constitutes a pair,as described above. The generated response frame may not include atransmission address.

On the other hand, a delayed response is used in the following manner: aresponse is made through an ACK frame as a basic response for informingof whether an initially generated request frame has been received ornot, and a response frame including requested information is thentransmitted. The response frame in this case may be transmitted throughEDCA (Enhanced Distributed Channel Access) regarding channels,piggybacked by another frame, or aggregated and transmitted togetherwith another frame. According to the delayed response scheme, atransmitting STA receives a response frame from a receiving STA and theninforms the receiving STA that the response frame has been receivedusing an ACK frame.

The request and response frame exchange sequence in such a WLAN may beapplied to a multiple user wireless communication system. When uplinkMU-MIMO technology is supported, wireless STAs can receive framesaccording to the above-mentioned frame exchange sequence andsimultaneously transmit response frames after IFS.

When uplink MU-MIMO technology is not supported, or when betterthroughput is desired even if uplink MU-MIMO technology is supported, awireless STA, after receiving a frame, needs to transmit a responseframe using a difference of channel or time.

Schemes for exchanging frames using a difference of time but the samebandwidth in a WLAN system include a scheme of exchanging a requestframe eliciting response frames and a response frame through channelaccess for each wireless STA, and a scheme of transmitting, by aplurality of wireless STAs, subsequent response frames through a singlerequest frame.

To be specific, the scheme of exchanging a request frame elicitingresponse frames and a response frame through channel access for eachwireless STA is as follows: an AP transmits a request frame to a firstwireless STA through channel access; the first wireless STA transmit aresponse frame after SIFS; the AP transmits another request frame to asecond wireless STA through channel access; and the second wireless STAtransmits a response frame after SIFS.

The scheme of transmitting, by a plurality of wireless STAs, subsequentresponse frames through a single request frame is as follows: a singlerequest frame including address information regarding multiple users istransmitted through channel access, and the wireless STAs thensubsequently transmit response frames after SIFS.

IEEE 802.11 standards recommend that, in order to avoid collisionbetween frames on a wireless path, control frames are transmitted to bereceivable by all STAs belonging to the corresponding BSS. In aMU-MIMO-based VHT WLAN system, control frames need to be transmitted tobe receivable by all STAs, even if a plurality of communication pathsare used. The control frames in this connection include RTS, CTS, ACK,BAR, BA, and various poll frames.

When a plurality of communication paths are used simultaneously, STAsreceiving control frames may have one-to-one correspondence torespective communication paths, meaning that that a control frame mustbe transmitted through each communication path. However, this increasesthe rate of overhead concerning transmission/reception of controlframes, degrading the advantage of using MU-MIMO technology.

DISCLOSURE Technical Problem

An embodiment of the present invention is directed to a method forgrouping a plurality of wireless STAs and transmitting frames, in orderto transmit control and data frames efficiently in a wirelesscommunication system using MU-MIMO technology, so that when a framefails to be transmitted, the transmission-failed frame can be recoveredeffectively.

Other objects and advantages of the present invention can be understoodby the following description, and become apparent with reference to theembodiments of the present invention. Also, it is obvious to thoseskilled in the art to which the present invention pertains that theobjects and advantages of the present invention can be realized by themeans as claimed and combinations thereof.

Technical Solution

In accordance with an embodiment of the present invention, a method forrecovering a response frame, which has failed to be received normally,among response frames to be received from a plurality of wirelessstations in a wireless communication system using multi-user multipleinput multiple output technology includes: (a) transmitting a framerequiring a response and containing group control information fordesignating the wireless stations as recipients; (b) subsequentlyreceiving first response frames from the wireless stations in responseto the frame requiring a response; (c) transmitting, when the firstresponse frame fails to be received normally from a specific wirelessstation in the step (b), a request frame having a wireless station, fromwhich the first response frame has failed to be received, as a recipientafter expiration of time for receiving the first response frames fromall of the wireless stations constituting a group; and (d) receiving asecond response frame in response to the request frame of the step (c).

In accordance with another embodiment of the present invention, a methodfor recovering a response frame, which has failed to be receivednormally, among response frames to be received from a plurality ofwireless stations in a wireless communication system using multi-usermultiple input multiple output technology includes: (a) transmitting aframe requiring a response and containing group control information fordesignating the wireless stations as recipients; (b) subsequentlyreceiving first response frames from the wireless stations in responseto the frame requiring a response; (c) transmitting, when the firstresponse frame fails to be received normally from a specific wirelessstation in the step (b), request frames having wireless stations asrecipients ranging from the wireless station, from which the firstresponse frame has failed to be received, to the last wireless stationbased on an order of transmitting the first response frames; and (d)receiving at least one second response frame in response to the requestframe of the step (c).

The group control information may include at least one of a groupsequence number uniquely assigned to each group, bitmap informationindicating whether wireless stations constituting a group have receivedor not, and stream information regarding multiple antennas.

The request frame may include the group sequence number and bitmapinformation having a bit set to indicate a reception state of a wirelessstation from which the first response frame has failed to be received.

The request frame may include the group sequence number and bitmapinformation having bits set to indicate reception states of wirelessstations ranging from the wireless station from which the first responseframe has failed to be received to the last wireless station.

The group control information may be recorded in a receiving stationaddress field of the frame requesting a response or the request frame.

The request frame may be a BAR (Block ACK Request) frame, the secondresponse frame may be a BA (Block ACK) frame, and the BAR frame mayinclude group information and receiving station information.

The group information may be recorded in a receiving station addressfield of the BAR frame, and the receiving station information may berecorded in a BAR information field of the BAR frame.

Advantageous Effects

According to the present invention, group sequence numbers are allocatedto a plurality of wireless STAs to group them, and the group sequencenumbers are used to transmit frames to the wireless STAs. Whentransmission of any of multiple frames using such group sequence numbersfails, efficient recover of the transmission-failed frame is supported.As such, the present invention substantially improves frame transmissionefficiency in a system using MU-MIMO technology.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the structure of a group control frame in accordancewith the present invention.

FIG. 2 illustrates the overall operation of a WLAN using a group numberin accordance with the present invention.

FIG. 3 illustrates a method for transmitting a group control informationframe in accordance with the present invention.

FIG. 4 illustrates a method for transmitting a frame in accordance withthe present invention.

FIG. 5 illustrates a method for recovering a CTS frame, which has failedto be transmitted, in accordance with an embodiment of the presentinvention.

FIG. 6 illustrates a method for recovering a CTS frame, which has failedto be transmitted, in accordance with another embodiment of the presentinvention.

FIG. 7 illustrates a method for recovering a block ACK frame, which hasfailed to be transmitted, in accordance with the present invention.

FIGS. 8 and 9 illustrate methods for recovering a block ACK frame, whichhas failed to be transmitted, using a group BAR frame in accordance withthe present invention.

FIG. 10 illustrates a group BAR using a multiple-TID BAR structure inaccordance with an embodiment of the present invention.

BEST MODE FOR THE INVENTION

Exemplary embodiments of the present invention will be described belowin more detail with reference to the accompanying drawings. The presentinvention may, however, be embodied in different forms and should not beconstructed as limited to the embodiments set forth herein. Rather,these embodiments are provided so that this disclosure will be thoroughand complete, and will fully convey the scope of the present inventionto those skilled in the art. Throughout the disclosure, like referencenumerals refer to like parts throughout the various figures andembodiments of the present invention.

The present invention will hereinafter be described with reference to awireless communication system, among various types of high-speedwireless communication systems, to which MU-MIMO technology is appliedbased on a wireless communication system defined by IEEE 802.11.However, those skilled in the art can understand that the presentinvention is applicable to any environment using multiple channels orany wireless communication environment using MU-MIMO technology.

A frame transmitted using MU-MIMO technology needs to includeinformation regarding multiple users' receiving STAs. Methods forconfiguring multiple receiving STA information in a frame include amethod of carrying a list of multiple users' receiving addresses insidethe transmitted frame, a method of carrying a group address inside thetransmitted frame, and a method of carrying, inside the transmittedframe, group control information including a group sequence number,bitmap information, and stream information so as to indicate a group andwhether a wireless STA has received or not.

Prior to describing a method for recovering a frame that has failed tobe transmitted, a method for transmitting multiple frames using groupcontrol information will now be described.

FIG. 1 illustrates the structure of a group control frame for groupcontrol information allocation in accordance with the present invention.

The group control information includes the number of multiple receivingSTAs, a group sequence number uniquely allocated to a group, bitmapinformation (STA bitmap) indicating whether reception has occurred ornot regarding wireless STAs constituting multiple users, and streaminformation. The multiple receiving STA information includes receivingSTA information, the number of which corresponds to that of the multiplereceiving STAs.

The number of multiple receiving STAs indicates the number of STAsdesignated as a group, and is not a necessary component, but an optionalone.

The group sequence number refers to a unique number managed by each APto group wireless STAs existing inside a BSS, and may have a length of 6bits, for example. If group numbers corresponding to 1-64 can beexpressed as a combination of bitmap information and stream information,the group sequence number may be omitted. In other words, the group'sunique number may be expressed as the location of at least one piece ofgroup control information obtained by coupling each bitmap informationand stream information. When a group sequence number is used in atransmitted frame, group control information (e.g. bitmap informationand stream information) lying at a location corresponding to anarbitrary group sequence number is included in the transmitted frame.

The STA bitmap information indicates whether each STA has received ornot according to the field order of n receiving STA information. Forexample, when the number of multiple receiving STAs is four (n=4),bitmap information “1101” indicates that frames are transmittedreceiving STAs 1, 2, and 4. Therefore, each STA, in the process ofhaving group control information allocated thereto, uses the address inthe receiving STA information field to recognize its location on thebitmap.

When one wireless STA is informed of a group sequence number to which itbelongs, i.e. when there is one wireless STA corresponding to a groupsequence number, 1-bit information (i.e. ‘1’ or ‘0’) is used to informof whether reception occurred or not. When a STA belongs to a pluralityof groups, which are described using a group sequence number, the STAbitmap information may be used to inform of whether reception hasoccurred or not regarding the corresponding group.

The stream information is used to indicate a stream location of atransmitting antenna in MU-MIMO environment. Specifically, the streaminformation indicates, when the AP uses four antennas, which of the fourantennas is used to transmit a frame. Therefore, a receiving STA canknow the location of a stream to receive based on the streaminformation. The stream information may include information regardingeach of as many as a streams. Alternatively, when as many as a streamsare classified into b groups, the value of b indicates the receivinglocation.

The receiving STA information includes receiving STA's addressinformation. Besides the receiving STA's address information, thereceiving STA information may further include QoS information and MCSinformation. The receiving STA address may be a MAC address of anassociation ID.

Such a group control frame may be transmitted to a plurality of wirelessSTAs according to a broadcasting method. Alternatively, the groupcontrol frame may be transmitted to each wireless STA according to aunicast method. In the latter case, the group control frame may onlyinclude a group sequence number, its own bitmap information, and streaminformation.

A frame, which is transmitted after group control information regardinga plurality of wireless STAs has been completely allocated, onlyincludes a group sequence number, STA bitmap information, and streaminformation. As a result, each wireless STA uses the allocated groupsequence number and bitmap information to recognize the recipient, anduses the stream information to receive a signal.

Meanwhile, one wireless STA may have at least one piece of group controlinformation allocated thereto. Specifically, the group control frame mayinclude bitmap information regarding a wireless STA corresponding toeach of at least one group sequence number, as well as streaminformation. The same frame format illustrated in FIG. 1 is used evenwhen one wireless STA has a plurality of pieces of group controlinformation allocated thereto.

FIG. 2 illustrates the overall operation of a WLAN using a group numberin accordance with the present invention. The group number may be agroup ID or a group sequence number.

When a group number is used, the overall operation of a WLAN includes astep of defining a group number and negotiating or allocating it, a stepof requesting sounding information and responding, a step oftransmitting MU-MIMO data, and a step of responding to the MU-MIMO data.Exchange of RTS/CTS frames in this case can be performed when channelprotection is considered necessary, e.g. before sounding or before thestep of transmitting MU-MIMO data.

When the step of defining a group number and negotiating or allocatingit is performed concurrently with the step of performing sounding, aframe transmitted for this needs to include at least one purpose,meaning that it is substantially ineffective in connection withoperation control and loss. Therefore, the step of defining a groupnumber and negotiating or allocating it and the step of performingsounding are rather performed independently than concurrently. It willbe assumed in the following description of the present invention thatthe step of defining a group number and negotiating or allocating it isperformed independent from the sounding step when wireless STAs withinthe BSS need to be grouped.

FIG. 2 illustrates the overall operation of a WLAN system based on anassumption that four receiving STAs' AIDs are used to designate a group,and the system has two groups.

Referring to FIG. 2, a group number (group ID or group sequence number)is defined, and the group number is designated to each STA throughnegotiation, or an arbitrary group number is allocated, and the groupnumber is used to request sounding at step S201.

In response to the sounding request, each wireless STA transmits an ACKframe based on the schedule after SIFS, so as to perform feedback inresponse to the sounding request, at step S202.

After receiving a response frame from the last wireless STA in responseto the sounding request, the AP, after SIFS, transmits a CTS-to-Selfframe as a NAV distribution process before transmitting a MU-MIMO dataframe, at step S203. The CTS-to-Self frame includes group controlinformation that has been updated based on feedback information receivedin the previous sounding process. The group control information isupdated based on each wireless STA's channel information, which isincluded in the feedback information, or network configurationinformation (e.g. connection/disconnection).

Such a CTS-to-Self frame informs a receiving MAC, including a wirelessSTA that has not partaken in reception, of the response order, and awireless STA supposed to receive data uses the response orderinformation to calculate the time to transmit a response frame.

After NAV distribution is completed, the AP transmits a MU-MIMO dataframe, including a group number, to a plurality of wireless STAs and,after SIFS, receives a response frame (ACK frame) from each wireless STAin response to the transmitted data frame.

The updated group control information may be transmitted while beingcontained not only in the CTS-to-Self frame, but also in a control frameof a MAC level, which can be decoded by all wireless STAs includinglegacy wireless STAs. Alternatively, the updated group controlinformation may include information regarding whether a wireless STA hasreceived at least one different piece of group information, as well asstream information, and provided to each wireless STA as a managementframe in a broadcasting or unicast type.

A case of a group control frame transmitted to a plurality of wirelessSTAs in a broadcasting type will now be described.

When a STA receives a group control frame, it determines that it is thedestination of the transmitted group control frame if its address isincluded in the receiving STA information field and if the bitcorresponding to it inside the wireless STA bitmap is set to ‘1’. Then,wireless STAs transmit a response (ACK frame) to the group control frameafter SIFS. Each STA performs a response subsequently.

An exemplary method for subsequently responding by respective wirelessSTAs is as follows: Each wireless STA transmits an ACK frame after aperiod of time obtained by multiplying the sum of ACK frame transmissiontime and SIFS by the number of bits in the STA bitmap, which precede itsturn and which is set as ‘1’. In other words, a wireless STA transmitsan ACK frame after all wireless STAs that have a bit value of ‘1’ in thebitmap before its turn have transmitted ACK frames.

FIG. 3 illustrates a method for transmitting a group control frame inaccordance with the present invention.

Referring to FIG. 3, four wireless STAs constitute a group, and thebitmap consists of four bits. The AP constructs a group control frameincluding a group sequence number, wireless STA bitmap information,stream information, and receiving STA information, and sets a NAVprotection value in order to safely receive ACK frames from multipleusers consisting of four wireless STAs.

Referring to FIG. 3, each wireless STA transmits an ACK frame at itsturn to respond to the group control frame. When a wireless STA hasresponded at its turn, the AP designates the corresponding bit of thebitmap as ‘1’ during MU-MIMO data frame transmission. However, when awireless STA has not responded, the AP designates the corresponding bitof the bitmap as ‘0’ during MU-MIMO data frame transmission.

FIG. 4 illustrates a method for transmitting a frame using group controlinformation in accordance with the present invention.

In general, a method for transmitting a frame by a WLAN includes stepsof RTS/CTS/DATA/ACK. Transmission of a MU-MIMO data frame includes thesame process.

Exchange of RTS/CTS frame is a function requested for the hidden-nodeproblem and channel protection in the case of a frame having data longerthan the RTS threshold. The RTS/CTS frame exchange process is stillneeded when transmitting data using MU-MIMO technology, and the RTSframe in this case includes group control information in the receivingSTA address field. The group control information includes at least agroup sequence number and bitmap information. When group controlinformation is recorded in the receiving STA address field in thismanner, a frame can be transmitted to multiple users without separateframe format modification or addition.

Wireless STAs having the same group sequence number transmit a CTS framein response to a RTS frame at their response turns.

After RTS/CTS frame exchange, the AP transmits a CTS-to-Self frame,which may include updated group control information.

The AP transmits a MU-MIMO data frame using the updated group controlinformation. Referring to FIG. 4, a total of four wireless STAsconstitute the group included in the group control information. However,after the group control information is updated, it is three receivingSTAs that receive the MU-MIMO data frame transmitted using thecorresponding group sequence number. In other words, the bit regardingthe third wireless STA in the STA bitmap of the group controlinformation is set as ‘0’ to have a bitmap of ‘1101’, thereby indicatingthat no data frame is transmitted to the third wireless STA.

Each wireless STA uses the updated group control information, which isincluded in the CTS-to-Self frame, to subsequently transmit a block ACKframe in response to the data frame.

The destination address of each A-MPDU of the MU-MIMO data frameincludes the MAC address of the corresponding wireless STA, andreception of data by multiple users is performed independently withregard to each beamformed A-MPDU. Grouping of wireless STAs supposed toreceive data using MU-MIMO is determined based on a combination havinggood performance during simultaneous data frame transmission usingMI-MUMO technology and a combination of QoS information of transmitteddata.

A method for recovering a frame, which has failed to be transmitted to aspecific user, when frames are transmitted to multiple users using theabove-mentioned group control information will now be described.

The present invention is applicable to cases in which response frames tocontrol frames, such as an ACK frame, a CTS frame, and a block ACKframe, have failed to be received normally.

FIG. 5 illustrates a method for recovering a CTS frame, which has failedto be received in response to a RTS frame, in accordance with anembodiment of the present invention.

The AP records group control information in the receiving STA addressfield of a RTS frame and transmits the RTS frame to a plurality ofwireless STAs that have been grouped. In this case, the AP transmits theRTS frame through EDCA.

As has been described with reference to FIG. 4, upon receiving the RTSframe including group control information in the receiving STA addressfield, the wireless STAs recognize the recipients using the groupsequence number and bitmap information, and subsequently respond withCTS frames. However, wireless station STA3 may fail to transmit a CTSframe at its turn as illustrated in FIG. 5 and, as a result, the APcannot receive a CTS frame from STA3. Consequently, there is a need fora method for recovering such a failure to receive a CTS frame from STA3.

Referring to FIG. 5, in accordance with an embodiment of the presentinvention, when a specific wireless STA fails to respond with a CTSframe, the channel is left unoccupied for a predetermined period oftransmission time corresponding to the turn of the wireless STA. All CTSframes from other wireless STAs are received, and the NAV protectioninterval expires. Thereafter, a RTS frame is again transmitted only tothe wireless STA, from which a CTS frame has failed to be received.

In other words, if STA3 illustrated in FIG. 5 fails to transmit a CTSframe, the channel is left unoccupied for a period of time, which isreserved for STA3 to transmit a CTS frame, and a CTS frame is receivedfrom STA4. In this case, other wireless STAs within the BSS are in theprocess of NAV protection by duration of a RTS frame. After receiving aCTS frame from the last STA, the AP retransmits a RTS frame only toSTA3, from which a CTS frame has failed to be received, after SIFS. Thebitmap of group control information included in the receiving STAaddress field of the retransmitted RTS frame has a bit, whichcorresponds to none other than STA3, set as ‘1’. It is also possiblethat the AP records the address of STA3 in the receiving STA address ofthe retransmitted RTS frame and then transmits the RTS frame.

FIG. 6 illustrates a method for recovering a CTS frame, which has failedto be transmitted, in accordance with another embodiment of the presentinvention. Specifically, FIG. 6 corresponds to a case of failedtransmission of a CTS frame regarding STA3.

Another embodiment of the present invention, which will now bedescribed, is different from the above-mentioned embodiment in which,when there is no response to a CTS frame, the channel is left unoccupiedfor a predetermined period of transmission time reserved for thecorresponding turn. To the contrary, when a CTS frame fails to bereceived at a predetermined turn and thus causes CTS timeout, the APretransmits RTS frames to all wireless STAs coming after the wirelessSTA, from which no CTS frame has been received, and preparing totransmit CTS frames.

In other words, when a CTS frame fails to be received from STA3 andresults in CTS timeout as illustrated in FIG. 6, the AP retransmits RTSframes to STA3 and STA4.

As such, another embodiment of the present invention is aimed at solvingthe hidden node problem, which may become severer when MU-MIMOtechnology is applied, even if other wireless STAs within the BSS areperforming NAV protection by means of RTS frames.

The bitmap of group control information included in the receiving STAaddress field of the RTS frame, which is retransmitted from the AP, hasbits, which correspond to none other than STA3 and STA4, set to ‘1’.

A method for recovering a BA frame, which has failed to be received inresponse to a frame transmitted using group control information will nowbe described. The method for recovering a CTS frame, which has failed tobe transmitted in response to a RTS frame, described with reference toFIGS. 5 and 6 is also applicable to a method for recovering a BA frame.

FIG. 7 illustrates a method for recovering a BA frame, which has failedto be transmitted, in accordance with an embodiment of the presentinvention.

According to the method for recovering a BA frame in accordance with anembodiment of the present invention illustrated in FIG. 7, after waitingfor all wireless STAs to transmit ACK frames, including the time fortransmission of BA frames that have failed to be received as in the caseof the CTS frame recover method described with reference to FIG. 5.Then, BAR frames are transmitted to wireless STAs, from which no BAframes have been received, to request them to transmit BA frames.

Specifically, FIG. 7 illustrates a case in which the AP transmits a dataframe (MU-MIMO A-MPDU) to STA1, STA2, and STA4 using group controlinformation and fails to receive BA frames from STA1 and STA2.

The AP then waits for a BA frame to be received from the last wirelessSTA (i.e. STA4), and transmits BAR frames to STA1 and STA2, which havefailed to transmit BA frames, through channel access.

The receiving STA address of the transmitted BAR frame is group controlinformation including a bitmap indicating STA1 and STA2, which arerequired to retransmit BA frames. It is also possible to record theaddress of wireless STAs, which have failed to transmit, in thereceiving STA address of the BAR frame and transmit the BAR frame toeach wireless STA.

The BAR control information and BAR information, which the BAR framebasically includes, concerns one receiving STA. Therefore, when groupcontrol information is used to provide a BAR function regarding multipleusers, and when multiple users who have received a data frame (MU-MIMOA-MPDU) have common information, the BAR control information and BARinformation fields are filled with common information. However, if eachof multiple users has a different BA-related value, the BAR controlinformation and BAR information fields are filled with null values.

When a data frame (MU-MIMO A-MPDU) including one piece of group controlinformation is transmitted in FIG. 7, the group control transmissionprocess is completed by receiving all BA frames from multiple users. Ifthere is no change in the multiple user group combination, the presentinvention can reconfigure the wireless STA bitmap and transmit the nextdata frame (MU-MIMO A-MPDU). As such, use of group control informationby the present invention makes it unnecessary to record all addressesfor multiple users in frames, thus reducing overhead of multiple userframes.

It has been assumed in the above-described embodiments of the presentinvention that group control information is recorded in the receivingSTA address field of the frame. However, those skilled in the art canunderstand that, instead of being recorded in the receiving STA addressfield, group control information may be included as control informationin various frames that can be exchanged between wireless STAs usingMU-MIMO technology.

Operation cases of containing control information in various frames willnow be described.

FIGS. 8 and 9 illustrate a method for recovering a BA frame, which hasfailed to be transmitted, using a group BAR frame in accordance with thepresent invention.

Referring to FIG. 8, a data frame is transmitted to multiple users, andBA frames fail to be received from STA2 and STA3. Then, the AP performsbackoff after expiration of time for transmission of BA frames by allwireless STAs, and transmits group BAR frames, specifically, to STA2 andSTA3, from which BA frames have failed to be received.

Referring to FIG. 9, a data frame is transmitted to multiple users, anda BA frame fails to be received from STA2. Then, the AP transmits agroup BAR frame after PIFS. The group BAR frame is a block ACK requestframe directed to STA2, from which a BA frame has failed to be received,and to ST3, which comes after STA2.

The group BAR frame in FIGS. 8 and 9 may include group information andreceiving STA information, and may be used to adjust the BA responseschedule according to radio environment.

Meanwhile, the group BAR frame can be constructed using a receiving STAaddress field and a BAR information field using a multiple-TID BAR type,among existing BAR frame formats, as illustrated in FIG. 10.

Referring to FIG. 10, the group BAR frame is constructed by recordinggroup information in the receiving STA address (RA) field of existingBAR frame format, and by recording each STA's TID information (AID) andBA SSN (Starting Sequence Number) in the BAR information field.

Such expression of a group BAR frame for group control using existingBAR frame format is based on the assumption that wireless STAs can knowthis from BAR control information, or a wireless STA, which hasrecognized that the receiving STA address is group information, canunderstand the frame format.

The above-described methods can also be embodied as computer programs.Codes and code segments constituting the programs may be easilyconstrued by computer programmers skilled in the art to which theinvention pertains. Furthermore, the created programs may be stored incomputer-readable recording media or data storage media and may be readout and executed by the computers. Examples of the computer-readablerecording media include any computer-readable recoding media, e.g.,intangible media such as carrier waves, as well as tangible media suchas CD or DVD.

While the present invention has been described with respect to thespecific embodiments, it will be apparent to those skilled in the artthat various changes and modifications may be made without departingfrom the spirit and scope of the invention as defined in the followingclaims.

INDUSTRIAL APPLICABILITY

The present invention is applicable to technology for recovering aframe, which has failed to be transmitted, among frames transmitted to aplurality of wireless STAs using group numbers in a MU-MIMO-basedwireless communication system.

What is claimed is:
 1. A method of transmitting in a wirelesscommunication system, the method comprising: transmitting, by atransmitter, a data frame to a plurality of receivers, the data frameincluding group control information having a group identifier and streaminformation; and receiving, by the transmitter, a first receptionacknowledgement for the data frame from a first receiver of theplurality of the receivers, wherein the group identifier indicates agroup to which the plurality of receivers belong, and wherein the streaminformation indicates at least one spatial stream for each of theplurality of receivers.
 2. The method of claim 1, wherein a number ofbits for the group identifier is
 6. 3. The method of claim 1, whereinthe transmitter is an access point (AP).
 4. The method of claim 1,further comprising: transmitting, by the transmitter, a blockacknowledgement request after receiving the first receptionacknowledgement.
 5. The method of claim 4, further comprising:receiving, by the transmitter, a second reception acknowledgement forthe data frame from a second receiver of the plurality of the receivers.6. A device in a wireless communication system, the apparatuscomprising: a processor; and a memory operatively coupled with theprocessor and storing instructions that when executed by the processorcauses to the device to: transmit a data frame to a plurality ofreceivers, the data frame including group control information having agroup identifier and stream information; and receive a first receptionacknowledgement for the data frame from a first receiver of theplurality of the receivers, wherein the group identifier indicates agroup to which the plurality of receivers belong, and wherein the streaminformation indicates at least one spatial stream for each of theplurality of receivers.
 7. The device of claim 1, wherein a number ofbits for the group identifier is
 6. 8. The device of claim 1, whereinthe transmitter is an access point (AP).
 9. The device of claim 1,wherein the memory stores instructions that causes to the device totransmit a block acknowledgement request after receiving the firstreception acknowledgement.
 10. The device of claim 9, the memory storesinstructions that causes to the device to receive a second receptionacknowledgement for the data frame from a second receiver of theplurality of the receivers.